1. Technical Field of the Invention
The present invention relates, generally, to alkali silicate glasses which are used as the basic glasses for the manufacture of selective filter glasses. More particularly, the present invention relates to the manufacture of selective filter glasses, colored with Co.sup.2+ or with Co.sup.2+ and Ni.sup.2+, which have transmission ranges primarily in the ultraviolet and visible spectral region between 280 and 500 nanometers (nm). These ion-colored glasses are imbued with the characteristic absorption bands of Co.sup.2+ and Co.sup.2+ together with Ni.sup.2+ and, therefore, depending upon the layer thickness, produce selective bell-shaped transmission bands, each surrounded by two stop ranges.
The most important characteristics of these so-called band pass filters, when the layer thickness is known, are the maximum of spectral transmission in the transmission band of .tau..sub.max and the half-wavelengths .lambda.'1/2 and .lambda."1/2 with a spectral transmission of .tau..sub.max /2. The mid-point between the half-wavelengths .lambda.'1/2 and .lambda."1/2 is called the mean wavelength .lambda..sub.m ; the distance from .lambda.'1/2 to .lambda."1/2 is known as the half-value width (HW). (See, FIG. 1.)
In research, engineering and medicine, these glasses are used, for example, as conversion filters in a narrow spectral region and, more particularly, are used in the filtering of UV light sources in the emission range from 200 to 1100 nm, with particular significance as pass filters in the ultraviolet spectral region from 250 to 400 nm and as stop filters in the spectral region of approximately 420 to 680 nm. The glasses can readily have compositions that assure high chemical resistance to acids, alkalis and water.